Development of nuclear radiation detectors for electromagnetic radiation such as X-rays and gamma-rays using gases, liquids and semiconductors, such as CdTe, CdZnTe (CZT), HgI2, TlBr, and others, currently underway in the National Laboratories, universities, and industry, is vital for many applications including medical imaging, astronomy, and homeland security.
Signals from radiation detectors arise due to motion of charge carriers after they are formed by incident radiation. This statement applies equally to gas-filled ion chambers, proportional counters, and semiconductor detectors, such silicon diodes and germanium spectrometers.
As soon as the incident radiation interacts with the detector material and charge carriers begin to move toward the electrodes, the output pulse starts to form. There is no delay before pulse onset due to carrier time of transport from their point of formation to the collecting electrode. When the carriers are created in a radiation interaction event they are collected at an electrode and the charge induction process ceases.
In semiconductor radiation detectors, defects and impurities in the crystal lattice tend to trap charge carriers. Crystal defects are the major factor limiting the performance of today's room-temperature semiconductor detectors, such as CdTe, CdZnTe, TlBr, and HgI2. Crystal defects that trap carriers lead to incomplete charge collection (ICC) and dead areas in detectors that inhibit device efficiency and energy spectral responses. Also, defects degrade the yield of the big crystals acceptable for device fabrication.
The specific effects of ICC events on the detector response (pulse-height spectra) depend on the amount of lost charge and the distribution of defects within the crystals. Some defects, e.g., Te inclusions, cause small fluctuations of collected charge, degrading device energy resolution. In contrast, some flaws, e.g., subgrain boundaries, entail significant charge loss that moves events from a photopeak area to a spectra's continuum, thereby lowering device photo efficiency and reducing the peak-to-Compton ratio.
One straightforward solution to these ICC problems is to grow large, high-purity, defect-free semiconductor crystals and to use specially selected crystal cuts with low concentrations of small defects and impurities. Producing such semiconductor crystal detectors is typically a time and energy intensive as well as a cost prohibitive proposition, which limits the viability of any business model.
Although this approach is acceptable for small detectors, it is too expensive when large, >10 cm3, crystals must to be used. The probability of large crystals having acceptable levels of defects throughout is very low, while their cost is high. Therefore, the method more likely of success is to exploit the commercially available imperfect semiconductor crystals in some manner to improve their performance.
Electronically rectifying distortion in detector energy spectra related to the ICC events caused by the point defects is known at least as far back as 1994. In the Nuclear Instruments and Methods in Physics Research A article “Correction of incomplete charge collection in CdTe Detectors,” Eisen and Horovitz describe a theoretical function for correcting the ICC by correlating the charge collected with the position of interaction. Monte Carlo methods, genetic algorithms, rise time discrimination (RTD), and other methods have also been used to compensate for ICC effects on measured detector energy spectra. However, the ICC events caused by big extended defects and contributing to the spectra's continuums could not be corrected by these approaches.